The invention is based on a device for measuring the mass of a flowing medium, also known as a flow rate meter. A device is already known (European Patent 0 458 998) that has a measuring element, accommodated in a measurement support, and a flow rectifier and a grid accommodated upstream of the measuring element. The flow rectifier is provided in order to generate as uniform a flow as possible over the entire inside cross section. The grid, permanently secured to the flow rectifier, is intended to create superfine eddies in the flow, in order to create the most constant possible flow conditions downstream of the grid, thereby stabilizing the measurement signal at the measuring element. The grid is a wire grid, which has individual wires woven into a meshlike structure. The wire grid has equidistant mesh widths, so that there are many flow openings all having the same flow cross section. If there is a severely impeded oncoming flow to the grid, which is characterized by a nonuniform distribution of velocity and major velocity gradients, however, the result is an uneven distribution of velocity, even downstream of the grid. Such a velocity distribution has a disadvantageous effect on the measurement accuracy of the measuring element, however.
In the known device, the grid is embedded in the heated state in a ring of the flow rectifier. Since the grid is made up of individual wires that are woven together into a mesh structure, the wires can shift somewhat relative to one another. Embedding the wire grid in the plastic has the disadvantage that a temperature change and/or aging of the plastic of which the flow rectifier is made can cause the wire grid to become dented or to sag. If the wire grid becomes dented or sags, the individual wires of the grid shift about, disadvantageously altering the characteristic curve of the measuring element. Moreover, securing the grid to the flow rectifier permanently has the disadvantage that only relatively complicated flow rectifiers, with grids of variable mesh width, can be combined with one another. There is also the risk that when the grid is embedded in the heated state in the ring of the flow rectifier, expelled plastic will remain in the flow rectifier, creating obstacles in the flow that can cause signal scattering, especially in mass production. Moreover, the provided embodiment of a ring that protrudes from a surface of the flow rectifier disposed at right angles to the flow direction, is relatively complicated from a production standpoint.
Another known option for securing the wire grid is to provide the wire grid with a flanged-over edge that has notches in which ribs can engage in order to position the wire grid. The mounting or flanging over of the edge of the wire grid is complicated from a production standpoint, on the one hand. On the other, the notches provided on the edge, because of unavoidable production variations, on being flanged over can be associated only relatively imprecisely with the flow openings of the wire grid, and accordingly a precise alignment of the flow openings of the wire grid with the openings of the flow rectifier is impossible.